Structural and Magnetic Properties of Ni-doped Yttrium Iron Garnet (Y 3 Fe 5-X Ni x O 12 ) Nanopowders Synthesized by Self-Combustion Method

2022 This research aimed to investigate the structural and magnetic properties of Yttrium Iron Garnet by Ni-doped using self-combustion method. Ni-doped directly changed the structure into Y 3 Fe 5-X Ni x O 12 (x=0.00, 0.02, 0.04, 0.06, 0.08). Self-combustion method was obtained by stirring raw materials at room temperature (27°–28°C) and heated at 150°C until combustion occurred. The samples were sintered at 900°C with 120 minutes holding time. The phase identification revealed the cubic structure of garnet phase with the crystallite size from 62.73–62.87 nm. The molecular bonding from molecular bonding displayed Ni-O and Fe-O bonds, while the magnetic properties shown the highest saturation magnetization of 27.04 emu/g in the sample with additional Ni x=0.02, the highest magnetic remenance of 16.09 in the sample Ni x=0.02, and the highest coercivity of 0.029 in the sample with Ni x=0.08. This research, by adding nickel element, shows that the coercivity of Y 3 Fe 5-X Ni x O 12 decreased when the particle size is increased. The increase in Ni concentration as doping material cause the double exchange interaction and affected the lattice parameter, molecular bond, and magnetic


Introduction
An improvement for a signal processing device is identified in a radar detecting tool, communication, and instrumentation using a microwave device 1 . Microwave devices often use ferrite material due to its high specific resistance, extraordinary magnetic flexibility, price, and performance.
A good ferrite material used in microwave devices is yttrium iron garnet (YIG), ferromagnetic, that has high electric resistivity, stable high radiation, and relatively low magnetization. YIG has a general crystal structure of A 3 B 5 O 12 with the specific formula of Y 3 Fe 5 O 12 2,3 . Yttrium iron garnet (YIG) is a cubic ferrite material with la3 ̅ d structure 4 . YIG itself consists of three garnet lattices: dodecahedra, octahedra, and tetrahedra 5 as shown in Figure 1. Previous research 6 revealed that another material such as nickel (Ni) can be added into yttrium iron garnet (YIG).
Nickel (Ni) is a ubiquitous doping material since it replaces the Iron (Fe) element in yttrium iron garnet (YIG) that causes a formulaic change from yttrium iron garnet (YIG) into Y 3 Fe 5-X Ni x O 12 , apart from omitting oxygen from the lattice. Oxygen lattice vacuum modifies some magnetic properties from the yttrium iron garnet (YIG) 7 . Thus, this research analyzed the magnetic property changes due to the modification in the oxygen lattice after nickel addition. Self-combustion is the most comfortable and a fast method that involves temperature to react and quickly synthesize the powder material. It is a process to create smooth powder by combustion in a heated solution containing a salt of the desired metal which has been mixed with the appropriate fuel. Self-combustion is capable of generating high-quality powder (nanometer-sized), which depends on the preparation and parameter. During self-combustion, which is influenced by the heat of the combustion process, physical properties are obtained by the material, including phase purity, particle size, surface area, and agglomeration 8 .
Other researcher varied the additional nickel (x = 0.05, 0.1 and 0.2) using citrate combustion method because it enhance the magnetic saturation and decrease the coercivity of YIG (Y 3 Fe 5 O 12 ) 9 . Another study of YIG using ball milling method was done by varying sintering temperatures that resulting the enhancement of magnetic saturation due to the grain size improvement 10 . However, this research observed the influence of nickel doped (x = 0.00, 0.02, 0.04, 0.06, 0.08) to the physical and magnetic properties of YIG. and distilled water mix. We prepared 50 ml of distilled water and measured the weight of raw material with the composition specified in Table 1. Then, we mixed them in a beaker glass and entered the self-combustion method. The substituted Yttrium Iron Garnet formula by Ni-doping has follow the super exchange theory brought by the work of Agarwal et al. 9 . The suitable formula is : 3  3  2  2  3 12

Material preparation
Where the brackets { } denote dodecahedral "c" sites, [ ] denote octahedral "a" sites, and ( ) denote tetrahedral "d" sites. The substitution of magnetic and non magnetic ions on YIG, distorts the crytal structure, Krishnan et al. described that Ni 2+ ions able to substitute both the tetrahedral and octahedral iron sites 11 . From the garnet structure, it is well known that an octahedral and a tetrahedral ion are linked with each other only once through the oxygen ion, that means each oxygen ion is coordinated with only one each of the octahedral and tetrahedral ions. Therefore, for each of the two octahedral iron ions there are six linkages with tetrahedral ions for a total of twelve, and for each of the tetrahedral ions there are four linkages with octahedral ions for a total of twelve. Consequently there are 24/5 linkages per iron ion in yttriumiron garnet 12 .

Self-combustion method
The self-combustion method was chosen to carry out this research due to its ease of operation and also more efficient synthesis process compared to co-precipitation, hydrothermal, and pulse laser deposition methods [13][14][15] . It was performed by dissolving the base material into the solvent with composition, as seen in Table 1. The combination was done using a magnetic stirrer at room temperature for 60 minutes and at 200 rpm speed. The solution was heated at 150 °C temperature until combustion in a beaker glass dried and turned into reddish-brown. The dried gel when heated in an open air underwent a self-propagating and intense exothermic reaction between nitrate and citrate ions 16,17 . The samples underwent a drying process at 150°C and crushed for 60 minutes. After crushing, the samples were sintered at 900 °C in 120 minutes holding time and experienced the second crushing for 60 minutes. The schematic diagram of Nitrate-Citrate mechanism shows in Figure 2.

Characterization
After obtaining the synthesized powder, the sample was characterized. Phase identification used the X-ray Diffraction

Phase characterization
Combined phase analysis used diffractogram values and data from the Crystallography Open Database (COD) with the database in this study based on COD-Inorg REV248644. The garnet structure has pure single-phase Y 3 Fe 5 O 12 and belongs to the Ia3d group 7,21 .
Highest peak on this XRD graph ( Figure 3) for all samples is about 32-33 2 theta, which means that (402) hkl index used as base crystallite size (D) calculation with Scherrer equation 22 .
where D is the mean crystallite size, k (0.89) is the Scherrer constant, λ is X-ray wavelength (0.154252 nm), and β is the relative value of the full width at half maximum (FWHM) of the diffraction peak (402). Apart from the phase, the  crystallite size was obtained from X-ray Diffraction (XRD) 23 . We obtained this value using the Scherrer Equation 24,25 . Ni concentration as doping nanoparticles, also called as the impurity to the host ions. To incorporate impurities into the host lattice, a common route is to adopt the doping ions with the sampe valence and the similar radii to the ones in the hosts. The possible changes in the crystal lattice after impurity doping will cause the crystal lattice expance 26 . Based on Table 2 27 . However, this is not applied since the sintering temperature is the same as each sample. The decrease in the crystallite size are attributed to the substitution of Fe cation by Ni ions. Ni ions are succesfully incorporated into the ferrite and restrict the grain growth which result the slightly decrease of crystallite size 28 .
An enlargement of the crystallite size occurred in the sample Y 3 Fe 4.94 Ni 0.06 O 12 , called a deviation. This was due to distortion as oxygen vacancies increase and reduce the Fullwidth half maximum (FWHM) value of the material. This causes the value of the peak position to be low compared to other samples. This is reinforced by the Scherrer equation where the crystallite size is inversely proportional to the peak position of the phase 29 . The value of lattice parameter gradually decrease with respect to Ni concentration which is attributed to the ionic radii of the respective ions ferrites. This result was consistent with the research of 30 Figure 4 shows that the addition causes the grains to aggregate but still spread evenly, and with this magnification, the shape and size of the sample grain can be seen. It is observed directly that the size comparison from Figures 3a to 3e has a relatively insignificant difference in grain size and has a different level of grain shape homogeneity. The difference in size is due to Ni-doped which has shown that Ni0.08 has the largest grain size, denser structure and fewer defect 32 .

Morphological characterization
The grain size produced in this synthesis can provide a nanometer size dominated by micrometer-sized granules. This result occurs in almost all samples and the grain sizes of nanometers also spread evenly. This is because nickel, which functions to replace iron, does not significantly change the morphology of yttrium iron particles 7 .

Fourier-transform infrared spectroscopy (FTIR) analysis
There are five functional group bonds with different regions in Figure 5 which are a graph of the Fourier-transform infrared spectroscopy (VS) results of yttrium iron nickel garnet. It shows that there are five functional group bonds in the Y 3 Fe 5-x Ni x O 12 sample (x = 0.00, 0.02, 0.04, 0.06, 0.08).
Sample is seen through a valley where the difference is not too significant. Region I contains a valley with each sample from Y 3 Fe 5-x Ni x O 12 (x = 0.00, 0.02, 0.04, 0.06, 0.08) 3446.79419 cm -1 , 3442.93656 cm -1 , 3442.93656 cm -1 , 3446.79419 cm -1 , 3442.93656 cm -1 . Previous research by Tholkapiyan et al. 30 uncovered that the wavelength of 3550-3200 is a bond of the O-H functional group with the stretching group, which has a strong and broad appearance that enters the alcohol group of compounds with intermolecular bonds. The frequency band in the range between 3200-3600 cm -1 is attributed to O-H stretching vibration of water molecules adsorbed during the exposure of dried powder to air.
Region III contains a valley with each sample from Y 3 Fe 5-x Ni x O 12 (x = 0.00, 0.02, 0.04, 0.06, 0.08) respectively 1637.56478 cm-1, 1631.77834 cm-1, 1631.77834 cm-1, 1637.56478 cm-1, 1639.4936 cm-1. It corresponds to the previous research by Adenkule et al. 33 who revealed that wavelength 1648-1638 is a a stretching band of C = C bond with a strong bond appearance and is included in the alkene class. This compound is derived from citric acid compounds as a combustion material that is not completely lost.
Region  34 showcases that the wavelength range of 610-664 cm -1 is a  36,37 . The bond of Fe-O and Ni-O compounds is the identity or fingerprint compound of the Y 3 Fe 5-x Ni x O 12 absorption band (x = 0.00, 0.02, 0.04, 0.06, 0.08). The reduction in the intensity peak and also the shifted of frequency because of the increasing Ni concentration. This is due to the Ni atoms occupying octahedral sites of Fe ions and pushing Fe ions towards oxygen ion 30 .

Magnetic properties analysis
The resulting hysteresis curve in Figure 6 has a narrow gap and a saturation value above 0.5 Oe (equivalent to 0.00005 T) 7 . Based on this statement, the Y 3 Fe 5-x Ni x O 12 sample (x = 0.00, 0.02, 0.04, 0.06, 0.08) is included in soft magnetic materials (Pena-Garcia et al. 38 ) Based on Figure 6, its magnetic characteristics can be analyzed using Magnetic Saturation (Ms), Magnetic remanence (Mr), and coercivity (Hc) presented in Table 3 which are obtained based on the approach of the hysteresis curve. The saturation magnetization (Ms) value for pure YIG nanoparticles was 23.684 emu/g. This is higher value than previously reported for DIG (8.29 emu/g) and Y3Fe5-xAlxO12 (6.97-0.308) 30 . The magnetic moment of Fe ions at the tetrahedral site is antiparallel and coupled to the octahedral site through super exchange interaction, wthich result the net magnetic moment. The increase of Ni concentration leading to increases of saturation magnetization (Ms) value because of a double exchange interaction. This interaction can induce cation ordered distribution in the octahderal site and resulting ferromagnetic behaviour 30 .    27 contended that the magnetic characteristics of a material are related to and influenced by the crystallite size of the material obtained from x-ray diffraction (XRD) through the Scherrer equation. However, this research shows that nickel doping has not effected the crystallite size value. Therefore, this section specifically explains the effect of nickel dopping on the Magnetic Saturation (Ms), Magnetic remanence (Mr), and coercivity (Hc) values in the Y 3 Fe 5-x Ni x O 12 material (x = 0.00, 0.02, 0.04, 0.06, 0.08).
Based on the comparison of Ms and H C to nickel composition, Ms value is inversely proportional to the value of Hc. This statement is supported by the Brown's Equation 39,40 .  Figure 7 presents the Ms value of raw material with Ms of 23.684 emu/g, while this research was to observe changes in samples doped with Ni. Ms reaches the maximum value in the sample x = 0.02 with a value of 27.039 emu/g, x = 0.04. It produces a value of 18.825 emu/g, x = 0.06 which is 26.924 emu/g and also the minimum value which is obtained at x = 0.08 is 2.518 emu/g. This value of Ms is generated from the anti-parallel coupling between the two magnetic sub-lattices for the octahedral (16 a-sites) and the tetrahedral (24 d-sites) positions filled with Fe 3+ ions. According to the analysis of Figure 8 the structure of Ni ion is located in the octahedral and tetrahedral sites. Theoretically, the substitution of iron with nickel at the site reduces the YIG total magnetic moment. This happened at x = 0.02, 0.04, and 0.08. Besides, Ni can also increase the value of Ms, such as the phenomenon at x = 0.06 due to the oxygen void caused by the presence of nickel. There is a strong spontaneous interaction between the oxygen vacancies and the magnetic ion in the octahedral lattice (Fe and Ni) that eliminates thermal effects and increases the sub-grid magnetic moment of the octahedral 38 . The very low Ms value at x = 0.08 with a value of 2.518 emu/g can be attributed to the spin canting mechanism where the substitution of Ni ions causes non-collinear spin changes, which lead to crystal lattice irregularities 7,25,38 . This causes some sites to have no ion interactions and causes the Ms value to decrease.
The magnetic remanence (Mr) value strongly depends on the Ni concentration and the same factor effect as the magnetic saturation (Ms). It can be seen in Figure 9 The fluctuation that occurs is similar to the state of Ms with a maximum remanence value of 16.04 emu/g at x = 0.02 and a minimum value of 1.13 emu/g at x = 0.08. Previous research explored the increase of Ni concentration and Ni atoms that can come close to each other. This implies the presence of some Ni 2+ ions with Ni 2+ nearest neighbors 26 . Increasing Ni concentration will increase the volume fraction of Ni 2+ ions. The super exchange interaction between these Ni 2+ ions is responsible for ferromagnetism 41,42,43,44 . Thus, the reduction of saturation and remanence magnetization in our samples are due to the enhanced antiferromagnetic interaction. Figure 10 shows that high coercivity (Hc) for x = 0.00 is 0.021 T and 0.02034 T for x = 0.02. The crystallite size for x = 0.00 is 62.74 nm and 62.73 nm for x = 0.02. Furthermore, the critical size for the single domain configuration at YIG is 35 nm, so that an increase in Hc is expected, but the opposite happens because the spin canting mechanism ( Figure 10) destroys the single domain configuration locally and causes a decrease in the coercivity value to 0.02034 T 7 . The Hc value of ferrite nanoparticles depends on the micro-strain, inter-particle interaction, magneto-crystalline anisotropy, particle size, and morphology 28, . The decreasing of Hc value with increasing Ni concentration due to the porosity. Porosity affects magnetization process because pores work as a generator of demagnetizing field. The porosity decrease leads to lower values of the coercive field 28,41 .
The same argument is applied to x = 0.04 and x = 0.06 where the sizes of 62.74 nm and 62.87 nm cause fluctuations of Hc to 0.0234 T and 0.021 T, with the crystallite size which is quite significant at x = 0.06. Another mechanism appears and causes fluctuation, magnetic moments that appear at x = 0.06. x = 0.08 indicates a decrease in the crystallite size to 62.73 nm and an increase in Hc value to 0.029 T. This finding is consistent with the statement that  when the crystallite size approaches the critical size, the Hc value increases 28,45 .

Conclusion
This research has successfully performed Y 3 Fe 5-x Ni x O 12 (x = 0.00, 0.02, 0.04, 0.06, 0.08) magnetic material synthesis based on the analysis and material testing using several characterizations. The XRD analysis also showed that the magnetic material has a single phase with the highest peak having an hkl index [402] with a position range of 32-33º. Besides, it has a crystallite size of nanometer size with a size range of 62.73-62.87nm. The grains obtained in this research are aggregated and have an even distribution. These can produce nanometer grain size, albeit micrometer-sized grains are dominating. Our research also successfully generated Fe-O bonds in the wavelength range of 648-694cm -1 and Ni-O in the wavelength range of 453-457cm -1 . These two bonds appear because Fe is the main element in the YIG material and Ni is its substitute. Based on the magnetic characteristics, the best samples which has excellent Ms, Mr, and Hc, is Y 3 Fe 5-x Ni x O 12 with Ni = 0.02.